1. Field of the Invention
The present invention relates to a differential drive semiconductor optical modulator, which is suitable for an optical modulator in optical transmission systems.
2. Description of the Related Art
In optical transmission systems, a optical modulator for modulating light based on an electric signal is used, and the signal transmission capacity can be improved by enhancing the modulation rate thereof.
The related prior arts are listed as follows:
[document 1] U.S. Pat. No. 5,930,022
[document 2] Japanese Patent Unexamined Publications (koukai): JP-2002-277840, A
[document 3] Japanese Patent Unexamined Publications (koukai): JP-2002-296552, A
[document 4] Hiroaki Takeuchi et al., “High-speed Electroabsorption Modulators with Traveling-Wave Electrodes”, OFC2002 Technical Digest WV1, 2002
The document 1 discloses an example where a differential amplifier is employed for a driver circuit of an electro-absorption (EA) type optical modulator, in which one drain of a differential FET is connected to a load resistor and another drain of another differential FET is connected to a parallel circuit including an EA modulator and a variable impedance circuit. However, since only one of two differential outputs is utilized for a driving signal and another of differential outputs is not utilized, resulting in a drawback in respect of power consumption and voltage use efficiency.
The document 2 discloses an optical module having a transmission-line substrate, semiconductor optical modulators and a terminator substrate mounted separately thereon, wherein these components are mutually wired.
The document 3 discloses an electro-absorption type optical modulator in which two electro-absorption type optical modulator are arranged in series along an optic axis and one optical modulator is supplied with an NRZ (non-return to zero) data signal and another optical modulator is supplied with a clock signal, thereby performing optically RZ (return to zero) coding.
The document 4 discloses that in case of a semiconductor optical modulator lengthened, degradation of the modulation band can be prevented by designing the characteristic impedance of the optical modulator at 50 ohms. However, it is difficult to manufacture such a modulator and attain a sufficient performance since the manufacturing process is super-fine.